Plate for offset printing and method for manufacturing said plate

ABSTRACT

The plate includes an electrochemically anodisable metallic support of a thickness of 0.1-0.6 mm, carrying on one of its side a ceramic film of multilayer structure. The plate has greater hydrophillicity, resistance to chemical oxidation and to mechanical abrasion, and a lower surface tension that those of the prior art.  
     The method comprises a first step in a bath containing sodium, potassium, ammonium or calcium borates, chlorides, carbonates and/or nitrates or forming mixed salts with elements of groups IIIB, IVB and VB, in a proportion of 5-40% by weight in relation to the total of the solution, at 20-60° C., 5-20 kC/m 2  and 50-250 V; and a second step in a bath containing sodium, potassium or lithium phosphates, silicates and/or carbonates, in a proportion of 1-35% by weight in relation to the total of the solution, at 20-70° C., 5-25 kC/m 2  and 150-400 V.

FIELD OF THE INVENTION

The present invention relates to a new plate for offset printing and to a method for manufacturing it. The plate according to this invention shows greater hydrophilicity and higher surface energy, and greater resistance to chemical oxidation and mechanical abrasion than the conventional plates. It also has excellent capacity for anchoring the light-sensitive varnishes placed on it.

The manufacturing method consists in a unified treatment with lower generation of waste effluents than the conventional treatments, thereby providing a surface with very different and advantageous properties.

BACKGROUND OF THE INVENTION

Nowadays, 95% of the printing forms for offset to be found worldwide have an aluminium base, which base is submitted to a series of physical-chemical treatments in order to make its characteristics better suited to its subsequent use. Of such treatments, the main ones are the following:

-   -   a) Degreasing;     -   b) Mechanical, chemical or electrochemical graining or buffing;     -   c) Anodising;     -   d) Hydrophilising and/or surface sealing.

These processes are carried out in baths of very different composition and under very different conditions of application, for which reason they require specific control and analysis systems.

Moreover, said processes generate effluents that have to be treated in a physical-chemical treatment system due to their contaminating charge, which is in turn basically due to their acidity or alkalinity and content in Al⁺³ ions.

Offset printing is to printing technique which is based on the different chemical affinity for water and grease (inks) of two surfaces arranged in the same plane (unlike typography), in such a way that the hydrophilic surface is covered with a permanent film of water, while the lipophilic surface (which forms the image to be printed) is covered with a film of ink, thus permitting transfer of the image formed on the offset plate to the paper, by means of an intermediate rubber roller.

The graining or buffing of the aluminium surface serves to increase the real surface area of the aluminium plate, thereby achieving better anchoring of the light-sensitive lacquer or varnish (which will be the lipophylic zone) and better retention of the water which will prevent the ink being deposited on undesired zones.

The anodising must protect mechanically (against scratches, uncontrolled episodes of oxidation and “flattening” of the “grain” inherent in the stresses of the printing machine) and must at the same time be capable of presenting good adherence for the light-sensitive film of varnish though without leading to undue retention of same in the non-printing (hydrophilic) following development.

All this is achieved by optimising the characteristics of both surfaces (hydrophilic and lipophylic) and an exhaustive control over production parameters.

Patent EP 0514312 in the name of Candela Munoz (16.05.91) proposes a ceramicisation procedure alternative to the conventional b) graining and c) anodising, which comprises the formation of a ceramic surface on the aluminium through the electrochemical reaction which occurs when the aluminium is treated in a bath which contains sodium and/or potassium silicates, in a proportion of between 4% and 30% by weight with respect to the total weight of the solution. The treatment takes place at temperatures between 10° C. and 50° C., using continuous current, consuming from 30 to 90 kC/m², so that a film is provided with a thickness of between 0.5 and 4.0 μm and an average roughness (Ra) of between 0.3 and 0.9 μm. Said coating is characterised in that it is inert, needs no subsequent sealing and is much harder and more abrasion resistant than the conventional anodised coatings.

However, the mechanical and chemical resistance of said film is only slightly higher than the conventional anodised surfaces when the thickness of the ceramicised layer is very high, thus presenting problems of retention of the light-sensitive coating or greasing by mechanical stamping of ink into the surface structure during use of the plate.

Another noteworthy disadvantage of the printing plate according to EP 0514312 is that the surface obtained in silicate baths has a very high pH, which leads to alteration of the light-sensitive compounds placed on the surface to form the offset plate, even after energetic processes of washing the plate, due to the three-dimensional porous structure which characterises said films, hindering elimination of the alkaline remains and greater the thicker the film formed.

Yet another disadvantage is the fact that due to the high surface porosity of the ceramic layers of silicates inks are attracted into their cavities when there is insufficient water on their surface, a situation which arises quite frequently in the printing process. Said ink is extraordinarily difficult to remove and often leads to the plate becoming useless due to a tendency to become greased (see FIG. 1).

Furthermore, a subsequent study carried out by the present applicant showed that it is not possible to develop ceramic films directly in sodium or potassium silicate baths, within the concentration limits stated in said European patent and for any initial state of the aluminium surface. It is only possible to develop the ceramic plate satisfactorily if the aluminium surface initially has a non-greasy protective film, which prevents its dissolution caused by the high pH of the solutions used in the first steps of the procedure. If this requirement is not complied with, then no ceramicisation takes places.

Also, in accordance with the process described in said European patent, if the aluminium surface has a zone protected by a greasy substance, no ceramicisation takes place on it unless the treatment is made enormously longer, in spite of which the surface is usually left non-uniform. From this it can be deduced that the absence of grease on the aluminium surface must be ensured before ceramicising, but if, for example, alkaline solutions are used for degreasing, the form of natural oxide is removed from the surface of the aluminium and the ceramic layer cannot be built up.

For this reason, the initial state of the surface of the aluminium to be treated has an enormous influence on the result of the process, and becomes a problem for achieving regularity in the final characteristics of the plates obtained with this method.

Moreover, the electrolyte used according to the description of the aforesaid patent is alkaline, and when the bath becomes exhausted as a result of deposition of SiO₂ on the surface of the aluminium, the concentration of NaOH or KOH increases and this means that the chemical aggressiveness of the bath increases and mounts a major attack on the aluminium itself. This means that there is Al⁺³ in the bath, and that some of the bath has to be restored often, thus producing more waste effluent which has to be treated before it is disposed of.

DESCRIPTION OF THE INVENTION

A first aspect of the present invention is to overcome the disadvantages mentioned by providing a plate for offset printing whose surface presents very good hydrophilicity and higher surface energy, as a result of its structure, which allows the contribution of water during its use to be reduced by up to 30%, and the isopropyl alcohol content in the wetting solution to be reduced by at least 50%. Said surface also has considerable mechanical strength, since up to 1,000,000 copies can be made, in relation to the ceramic substrate. The high chemical resistance shows itself in the resistance of its surface to uncontrolled oxidation, so that the protecting rubber on the plate is often unnecessary. It also has very good capacity for anchoring the light-sensitive varnishes due to the three-dimensional crosslinked structure, presenting an average roughness of the ceramic layer (Ra) from 0.3 to 0.9 μm.

Advantageously, the new plate for offset printing of the present invention has a surface appropriate for thermal CTP (Computer to Plate) due to the low thermal conductivity and to the high melting point of the ceramic layer obtained.

In accordance with the first aspect, the present invention provides a plate for wet-process offset printing characterised in that it comprises an electrochemically anodisable metallic support with a thickness of between 0.1 mm and 0.6 mm, with at least one of its sides supporting a ceramic film of multi-layer structure which comprises:

(i) a first layer formed by a chemical structure derived from one or more of the following refractory oxides: SiO₂, Al₂O₃, TiO₂, HfO₂, BeO and ZrO₂, said first layer having a thickness of between 20 nm and 2 μm;

(ii) a second layer formed by the combination of at least one of the refractory oxides described in (i) with carbonatos and/or nitrides of elements of groups IIIB, IVB and IIIA of the periodic table, said second layer having a thickness of between 0.1 μm and 3 μm;

(iii) a third layer formed by the combination of one or more silicates of the elements of groups IA, IIA, IIIA and IVB of the periodic table, said third layer having a thickness of between 0.5 μm and 10 μm;

(iv) a fourth layer formed by oxides and/or carbonates of groups IVB and VB of the periodic table, said fourth layer having a thickness of between 50 and 600 nm; and

(v) a fifth layer formed by sulphates, carbonates, nitrates, phosphates and/or silicates of groups IA and IIA of the periodic table, said fifth layer having a thickness of between 10 and 500 nm.

In the present invention an “electrochemically anodisable metallic support” is taken to mean a sheet of aluminium, zirconium, aluminised steel, titanium or any other electrochemically anodisable surface.

In a preferred embodiment of the present invention the electrochemically anodisable support is a sheet or strip of aluminium with a degree of purity not less than 80%.

Advantageously, the multilayer structure of the ceramic film according to the invention has the following limits of thicknesses and composition:

(i) from 100 to 500 nm basically composed of aluminium oxide;

(ii) from 1 to 3 μm which includes at least 5% zirconium;

(iii) from 1 to 4 82 m composed of silicon oxide and silicates of the elements of groups IA, IIA, IIIA or IVB of the periodic table;

(iv) from 200 to 600 nm composed of zirconium oxide and/or carbonate;

(v) from 100 to 300 nm composed of acid phosphates of alkaline elements.

Said first and second layers can be formed easily, giving rise to a compact and very hard film, so that where the electrochemically anodisable support is a sheet or strip of aluminium the presence of an initial protecting film is not necessary. During formation of these two layers no appreciable attack or dissolution takes place, due to the nearly neutral, slightly alkaline pH of the bath.

Said third layer presents suitable porosity and average roughness, without exhaustion of the bath arising during its formation and involving an increase in its aggressiveness. Dissolving of the aluminium is thus prevented, and it is not therefore necessary to restore the bath frequently, so little waste effluent is produced.

The third, fourth and fifth layers are for controlling surface porosity, adherence and hydrophilicity.

Advantageously, the formation of said multilayer structure according to the invention presents a resistance to mechanical abrasion very much greater than that of the plate printing closest to it in the state of the art disclosed in EP 514312, thus permitting an increase in the useful life of the material of up to 1,000,000 printings in terms of the substrate.

The new plate for offset printing according to the invention also presents higher hydrophilicity and higher surface energy, together with good capacity for anchoring of the light-sensitive varnishes with which these surfaces are coated.

The chemical and structural nature of the surface of the plate of the invention confers greater storage capacity and fast and uniform distribution of water over its entire surface, which means that the supply of water to the offset machine can be reduced by up to 30%, thus providing sharper printer and stronger contrasts, since the content of water in emulsion with the ink in the image zones (the cause of loss of image) is reduced. There is a possibility also of reducing the quantity of isopropyl alcohol used in the wetting water, by at least 50%, also bringing an added ecological benefit.

Furthermore, and given that the plate for printing in accordance with the invention has new surface properties, its use is not limited only to a base for positive plates, to which positive light-sensitive varnishes are applied, but also negative plates with negative light-sensitive varnishes, white plates (without any type of varnish), plates with silver halide, photopolymer, etc., and even CTP (Computer to Plate) plates.

Especially advantageous is its application on thermal CTP plates, particularly the latest-generation ones, which following exposure with the laser system need no treatment or development before they can be used on the printing machine, unlike the standard plates.

An important property in this respect is the low thermal conductivity of the ceramic surface, which permits the definition of the points to be increased and therefore also the definition of work on the thermal CTP plates, in addition to permitting the user of lasers of lower power.

Especially advantageous is the fact that the shelf-life of thermal CTP plates is higher than those corresponding to conventional anodised substrates.

A second aspect of the present invention is to provide a method for obtaining a plate for offset printing with the characteristics described previously, by means of a treatment which permits continuous production and which ensures stability in the properties of the ceramic film.

With the method of the invention less dissolution of the aluminium takes place, while the pH of the effluents, if any, is much more neutral than those of the conventional offset plate production lines. Moreover, with the method of the present invention, the usual degreasing, graining or buffing and anodising treatments are replaced by a unified treatment, preferably carried out in two steps, to provide a surface of very different and advantageous properties.

The method of the invention for making a plate for offset printing allows the application properties of the end product to be improved, while at the same time reducing the generation of waste products and the toxicity thereof.

For this purpose, the invention proposes a method for manufacturing plates for wet-process offset printing characterised in that the ceramicisation of an electrochemically anodisable metallic support is carried out in an electrolytic treatment in two steps:

(i) a first step in a bath which contains sodium, otassium, ammonium or calcium borates, chlorides, carbonates and/or nitrates or forming mixed salts with elements of groups IIIB, IVB and VB of the periodic table, in a proportion of between 5 and 40% by weight in relation to the total of the solution; and

(ii) a second step in a bath which contains sodium, potassium or lithium phosphates, silicates and/or carbonates in a proportion of between 1 and 35% by weight in relation to the total of the solution.

Advantageously, the first step is carried out in a proportion of between 10 and 30% by weight in relation to the total of the solution and the second step is carried out in a proportion of between 3 and 30% by weight in relation to the total of the solution.

The utilisation, in the first place, of a bath such as that described for the first step, allows the formation of the first two layers without surface attack of the metallic support taking place, especially in the case of an aluminium sheet or strip. This bath is very easy to regenerate and its exhaustion does not increase its aggressivity towards the aluminium or give rise to great changes of pH. In principle, this bath does not require changing but only addition of the reagent consumed.

Furthermore, this first layer is very compact, hard and uniform (there do not arise inequalities which can affect the reactivity of certain zones of the aluminium). The hardness of this layer lends the plate great wear resistance. The compact appearance of the film makes the plate very resistant to corrosion.

The use of the bath described for the second step allows the formation of a ceramic layer, superimposed on the preceding ones, that present a porosity and average roughness suitable for using as a substrate for offset plates. As the bath is exhausted the pH of the solution increases. However, although the aggressivity of the bath increases, this does not directly affect the surface of the aluminium which is being treated as it is protected with the films formed in the first step, thus preventing dissolution of aluminium, so that the time when part of the bath has to be regenerated is postponed.

Advantageously, the addition of acid sulphates and/or phosphates of alkaline elements allows, on the one hand, correction of the excess concentration of OH⁻ in the bath and, on the other hand, allows ceramicisation to be carried out at very much lower voltages, with the attendant saving of energy.

The ceramicisation treatment for the first step takes place at temperatures of between 20 and 60° C. with surface density of charge between 5 and 20 kC/m² and a maximum voltage between 50 and 250 V, preferably between 80 and 150 V using direct current. The ceramicisation treatment for the second step takes place at temperatures of between 20 and 70° C. with surface density of charge between 5 and 25 kC/m² and a maximum voltage between 150 and 400 V, preferably between 180 and 300 V using direct current.

An especially notable feature of the present invention, in relation to the method for manufacturing plates for offset printing, is the fact of using tanks of special design for the ceramicising process, which allow the surface of the aluminium to be treated to be submitted to a variable and increasing potential according to an ideal preset pattern, until the desired maximum potential for each of the steps is reached (see FIG. 2). Without control over the potentials applied throughout these processes it is not possible to obtain uniform ceramic films suitable for use in plates for offset.

Advantageously, cathodes which present at least the same useful surface area as that of the aluminium treated in each bath, together with a design which allows rapid evacuation of gases and fast flow of the bath through them without setting up great turbulence will be used.

Also noteworthy is that fact that ceramicisation of only one of the sides of the strip of aluminium treated is achieved, leaving the opposite side protected but not ceramicised.

Advantageous results are achieved if pulsing direct current is used, as this allows different granular distributions of the ceramic surface to be obtained, as well as energy saving due to the lower electrical resistance found on the ceramicised surface as a result of the depolarisation undergone during the electrical relaxation times.

A sealing treatment is then carried out in order to avoid the tendency of the ceramic film to retain the emulsion excessively, and in order to be able to control at the same time the final porosity of the ceramicised surface. Said sealing treatment is carried out using a bath formed by dissolution of sodium, potassium, ammonium phosphates, silicates and/or carbonates of elements of groups IIIA, IVB and/or VB of the periodic table.

Advantageously, said solution is applied on the ceramicised surface at temperatures of between 10 and 50° C., preferably between 20 and 40° C., unlike the usual sealing treatments which generally require temperatures of between 50 and 90° C.

The concentrations of the bath are between 1 and 50% by weight in relation to the total of the solution, preferably between 5 and 30% by weight. This sealing treatment can be completed by precipitation by thermal dehydration on the ceramic surface.

Also advantageously, the plate obtained in accordance with the method of the invention is submitted to an adjusting ( ) treatment following the sealing treatment, which adjusts the final pH of the surface so that the light-sensitive varnish placed on it is not altered, that is, the problems of insolubilisation of the light-sensitive varnish by prolonged contact with excessively alkaline substrates are avoided. Said neutralising treatment comprises washing the plate with an aqueous solution of an acid or mixture of several acids selected from among the following: acetic acid, oxalic acid, phosphoric acid, citric acid and/or lactic acid, preferably phosphoric acid and/or citric acid in a concentration of between 1 and 40% by weight in relation to the total of the solution. Preferably, the concentration is between 5 and 30% by weight.

The adjusting ( ) treatment is carried out at a temperature between 10 and 50° C., preferably between 20 and 40° C.

DESCRIPTION OF THE FIGURES

FIG. 1 shows schematically the porous structure of a cross-section of a plate for offset printing of the prior art. In FIG. 1A a very porous surface is shown with high water-storage capacity, while FIG. 1B shows the remains of ink in the deepest hollows of the porous structure, which cannot be cleaned in a definitive manner.

FIG. 2 shows schematically a preferred embodiment of a tank for continuous-process ceramicising in accordance with the method of the present invention. As said Figure shows, as it advances the surface of the strip is submitted to the same density of current and therefore the electrical treatment applied is exactly the same as if the process had been carried out discontinuously at constant electric current. Each point of the aluminium strip throughout its entire travel is therefore submitted to a certain voltage so that with each travel speed and surface density of charge a voltage curve is obtained. In consequence, when it comes to designing the tank account must be taken of the speed at which ceramicisation is to be carried out, in order to determine the length of the tank in function of that speed.

PREFERRED EMBODIMENT OF THE INVENTION

In one embodiment of the invention, the multilayer structure comprises a first layer of aluminium oxide, a second layer of a component which comprises at least 5% zirconium, a third layer which comprises silicon oxide and silicates of elements of groups IA, IIA, IIIA or IVB of the periodic table, a fourth layer which comprises oxides and/or carbonates of zirconium and a fifth layer which comprises acid phosphates of alkaline elements.

In one embodiment of the method for making an offset plate offset according to the invention, in the first place a bath is used which contains ammonium-zirconium carbonate in order to form the second ceramic layer without an attack on the surface of the aluminium taking place, since the pH of the bath is nearly neutral. As the zirconium oxide is deposited on the surface of the sheet of aluminium, the bath becomes enriched with ammonia, a base which is much weaker than the sodium or potassium hydroxide of the prior art.

Moreover, the formation of a layer of zirconium oxide allows a very compact and hard film to be obtained, which on the one hand avoids the problems of surface inequalities in terms of reactivity and, on the other hand, increases the surface hardness and therefore brings a spectacular lengthening of the useful life of the product while also increasing the impermeability of the substrate.

Advantageously, the excess of ammonia is corrected by addition of CO₂ to once again form ammonium carbonate which regenerates the bath. In principle, this bath does not require changing, but simply addition of the zirconium or ammonium carbonate which is consumed.

In addition to being harder, the film of zirconium oxide protects the non-printing zone from possible wear and corrosion defects.

Advantageously, the ammonium—zirconium carbonate used is temperature stable, that is, it resists without decomposing, preferably for 24 h at 70° C.

Secondly, a bath which contains sodium silicate allows the formation of a third ceramic layer of suitable porosity and average roughness. As the silicon oxide is deposited on the surface of the layer of zirconium oxide the bath becomes enriched with sodium hydroxide. However, although the aggressiveness of the bath increases, this does not directly affect the surface of the aluminium, and therefore avoids a possible attack on same, thereby slowing down the regeneration time of part of the path.

Advantageously, the addition of diacid sodium phosphate on the one hand corrects the excess concentration of OH⁻ which takes place in the bath, and on the other hand allows the ceramicisation to be carried out at very much lower voltages, with the consequent saving of energy involved.

In one embodiment of the invention the ceramicising treatment in the first step was carried out at 30° C. with a charge density of 10 kC/m², and in the second step at 30° C. with a charge density of 15 kC/m².

Then, and in order to prevent the tendency of the ceramic film of silicate to retain the emulsion and the ink excessively its surface, the sealing treatment is carried out using a solution of ammonium—zirconium carbonate prior to a concentration of between 5 and 30% by weight in relation to the total of the solution and at a temperature between 20 and 40° C.

The plate is then submitted to an adjusting treatment which adjusts the final pH of the surface of the plate for printing, so that the light-sensitive varnish which will be placed on it does not alter. The plate is washed with an aqueous solution of a weak acid, preferably phosphoric acid at a concentration of approximately 25% in relation to the total of the solution and at a temperature of 30° C.

EXAMPLES Example 1

A strip of aluminium with a thickness of 0.14 mm and a width of 400 mm is taken. It is made to advance constantly at 5.0 m/min, while the anode is kept connected, through a first tank (first step of the treatment) where the bath is made up of a solution of potassium phosphate acid with a concentration of 10% by weight, ammonium—zirconium carbonate with a concentration of 19% by weight and ammonium—hafnium carbonate at 1% by weight in relation to the total of the solution.

The temperature of the bath is kept constant at 30° C. by means of a conventional heat evacuation system.

The strip of aluminium is gradually submitted, over the course of its travel through said tank, to a steady increase of voltage from 0 to 140 V, and from 120 V a clear white bubbling on the surface of the strip can be observed. When the treatment has been completed, the charge passed per unit of surface treated is 20 kC/m². The resulting surface is whitish and uniform.

The strip is then washed with abundant deionised water. Next it reaches the tank for the second treatment in which the bath is composed of 20% by weight of sodium silicate (38° Be) with a ratio of SiO₂/Na₂O of approximately 3, together with acid phosphate of sodium at 3% by weight in relation to the total of the solution.

The temperature of the bath is kept constant at 30° C. by means of a conventional heat evacuation system.

The strip is gradually submitted, over the course of its travel through said tank, to a steady increase of voltage from 140 to 280 V, and an orange-colour bubbling can be observed on the surface of the aluminium from 220 V. When the treatment has been completed, the charge passed per unit of surface treated is 25 kC/m². The appearance of the surface is matt and white.

The strip is washed with abundant deionised water. Next it goes to the sealing tank, whose bath is made up of 5% by weight of potassium silicate 42.5/43 and of acid phosphate of potassium at 1% by weight in relation to the total of the solution.

The temperature of the bath is kept constant at 30° C. by means of a conventional heat evacuation system.

The strip is then left for liquid to run off and is dried with the help of hot air at 60° C.

Finally, the strip passes through a solution at 10% by weight of phosphoric acid and citric acid at 2% by weight in relation to the total of the solution. The temperature of the bath is kept constant at 30° C. means of a conventional heat evacuation system.

Finally, the strip is rinsed with deionised water and dried. It is lacquered with positive light-sensitive varnish and is dried in an oven at 125° C. for 60 seconds.

Sheets of format 510×400 mm were cut from the ceramicised strip. There were exposed and developed. This resulted in 40,000 copies of high quality, having used 18% less water than usual and without using any of isopropyl alcohol in the wetting solution. Said plate was left to dry in the air for 5 hours, without gumming, after which 10,000 prints of high quality were obtained.

Example 2

A procedure similar to that described in example 1, but the composition of the bath of the first step was 15% by weight of ammonium—zirconium carbonate, 1% by weight of ammonium—hafnium carbonate and sodium tetraborate at 1% by weight in relation to the total of the solution. The need to provide 20% less water than usual was observed. 10,000 prints were made. Excellent results were obtained in the printing tests.

Example 3

A procedure similar to that described in example 2, but the composition of the bath of the first step was 10% by weight of acid carbonate of ammonium, 10% by weight de aminoformiatoammonium and ammonium—zirconium carbonate at 6% by weight in relation to the total of the solution. The speed of the strip was 3.0 m/min. The need to provide 10% less water than usual was observed. 25,000 prints were made. Excellent results were obtained in the printing tests.

Example 4

A procedure similar to that described in example 3, but the composition of the bath of the second step was 20% by weight of sodium silicate (38° Be) and trisodium phosphate at 1% by weight in relation to the total of the solution. In the first step 10 kC/m² were applied, and in the second step 8 kC/m². The speed of the strip was 1.5 m/min. The need to provide 10% less water than usual was observed. 10,000 prints were made. Excellent results were obtained in the printing tests.

Example 5

A procedure similar to that described in example 4, but the composition of the sealing bath was 5% by weight of acid carbonate of ammonium, 5% by weight of aminoformiateammonium and phosphate acid of sodium at 1% by weight in relation to the total of the solution. 20,000 prints were made. Excellent results were obtained in the printing tests.

Example 6

A procedure similar to that described in example 5, but the composition of the bath of the first step was 5% by weight of sodium carbonate, 19% by weight of ammonium—zirconium carbonate and ammonium—hafnium carbonate at 1% by weight in relation to the total of the solution. The sealing treatment was carried out in a bath composed of 8% by weight of potassium silicate 42.5/43 and acid phosphate of potassium at 2% by weight in relation to the total of the solution. The speed of the strip was 6.5 m/min. The need to provide 20% less water than usual was observed. 40,000 prints were made. Excellent results were obtained in the printing tests.

Example 7

A procedure similar to that described in example 6, but the lacquer used was of positive thermal-sensitive type, the drying of the lacquered plate was carried out at 90° C. for 150 seconds. After a maturation treating for 24 hours at 50° C., a plate sensitive to 830 nm laser radiation was obtained. It was exposed to a platesetter (available from Lüscher). It was developed at 23° C. with a conventional positive developer into a processor at a speed of 600 cm/min. 15000 prints were made with the resulting plate.

An excellent quality of reproduction was obtained.

In spite of the fact that one specific embodiment of this invention has been described and shown, it is obvious that an expert in the matter would be able to introduce variants and modifications, or replace the details by others technically equivalent, without departing from the sphere of protection defined by the attached claims. 

1. Plate for wet-process offset printing characterized in that it comprises an electrochemically anodisable metallic support with a thickness of between 0.1 mm and 0.6 mm, with at least one of its sides supporting a ceramic film of multi-layer structure which includes: (i) a first layer formed by a chemical structure derived from one or more of the following refractory oxides: SiO₂, Al₂O₃, TiO₂, HfO₂, BeO and ZrO₂, said first layer having a thickness of between 20 nm and 2 μm; (ii) a second layer formed by the combination of at least one of the refractory oxides described in (i) with carbonates and/or nitrides of elements of groups IIIB, IVB and IIIA of the periodic table, said second layer having a thickness of between 0.1 μm and 3 μm; (iii) a third layer formed by the combination of one or more silicates of the elements of groups IA, IIA, IIIA and IVB of the periodic table, said third layer having a thickness of between 0.5 μm and 10 μm; (iv) a fourth layer formed by oxides and/or carbonates of groups IVB and VB of the periodic table, said fourth layer having a thickness of between 50 and 600 nm; and (v) a fifth layer formed by sulphates, carbonates, nitrates, phosphates and/or silicates of groups IA and IIA of the periodic table, said fifth layer having a thickness of between 10 and 500 nm.
 2. Plate for printing according to claim 1, characterised in that said electrochemically anodisable metallic support is selected from a sheet or strip of aluminium, zirconium, aluminised steel, titanium or other electrochemically anodisable surface.
 3. Plate for printing according to claim 2, characterised in that said electrochemically anodisable support is a sheet or strip of aluminium with a degree of purity not less than 80%.
 4. Plate for printing according to claim 1, characterised in that said multilayer structure has the following limits of thicknesses and composition: (i) from 100 to 500 nm basically composed of aluminium oxide; (ii) from 1 to 3 μm which comprises at least 5% zirconium; (iii) from 1 to 4 μm composed of silicon oxide and silicates of the elements of groups IA, IIA, IIIA or IVB of the periodic table; iv) from 200 to 600 nm composed of zirconium oxide and/or carbonate; (v) from 100 to 300 nm composed of acid phosphates of alkaline elements.
 5. Method for manufacturing a plate for wet-process offset printing characterised in that the ceramicisation of an electrochemically anodisable metallic support is carried out in an electrolytic treatment in two steps: (i) a first step in a bath which comprises sodium, potassium, ammonium or calcium borates, chlorides, carbonates and/or nitrates or forming mixed salts with elements of groups IIIB, IVB and VB of the periodic table, in a proportion of between 5 and 40% by weight in relation to the total of the solution, at a temperature of between 20 and 60° C. with a surface density of charge between 5 and 20 KC/m² and at a maximum voltage of between 50 and 250 V; and (ii) a second step in a bath which comprises sodium, potassium or lithium phosphates, silicates and/or carbonates in a proportion of between 1 and 35% by weight in relation to the total of the solution, at a temperature of between 20 and 70° C. with a surface density of charge between 5 and 25 KC/m² and at a maximum voltage of between 150 and 400 V.
 6. Method according to claim 5, characterised in that in said first step the compounds are in a proportion of between 10 and 30% by weight in relation to the total of the solution.
 7. Method according to claim 5, characterised in that in said second step the compounds are in a proportion of between 3 and 30% by weight in relation to the total of the solution.
 8. Method according to claim 5, characterised in that said first step is carried out in direct current at a maximum voltage of between 80 and 150 V.
 9. Method according to claim 5, characterised in that said second step is carried out in direct current at a maximum voltage of between 180 and 400 V.
 10. Method according to claim 5, characterised in that a sealing treatment is then carried out in a bath formed by dissolution of sodium, potassium, ammonium phosphates, silicates and/or carbonates of elements of groups IIIA, IVB and/or VB of the periodic table at a concentration of between 1 and 50% by weight in relation to the total of the solution and at temperature between 10 and 50° C.
 11. Method according to claim 10, characterised in that said compounds are present in a concentration between 5 and 30% by weight in relation to the total of the solution.
 12. Method according to claim 10, characterised in that said sealing treatment is carried out at temperature between 20 and 40° C.
 13. Method according to claims 5 and 10, characterised in that a neutralising treatment is then carried out which comprises washing the plate with an aqueous solution of an acid or mixture of several acids selected from among the following: acetic acid, oxalic acid, phosphoric acid, citric acid and/or lactic acid, preferably phosphoric acid and/or citric acid in a concentration of between 1 and 40% by weight in relation to the total of the solution and at a temperature between 10 and 50° C.
 14. Method according to claim 13, characterised in that said acid or mixture of acids is selected from between acetic acid, oxalic acid, phosphoric acid, citric acid and/or lactic acid at a concentration between 5 and 30% by weight in relation to the total weight of the solution.
 15. Method according to claim 14, characterised in that said acid is selected from between phosphoric acid and/or citric acid.
 16. Method according to claim 13, characterised in that said neutralization treatment is carried out at a temperature between 20 and 40° C.
 17. Method according to claim 5, characterised in that organic or inorganic salts are added in each of the steps, preferably sulphates and/or acid phosphates of alkaline elements in order to reduce the maximum operating voltage.
 18. Method according to claim 5, characterised in that the method of ceramicisation is carried out using pulsing direct current.
 19. Method as claimed in any of claims 5 to 18, characterised in that said electrochemically anodisable support to be ceramicised is submitted to rising electrical voltages when said support travels though the treatment tank. 